The invention relates to block copolymers comprising at least two hard blocks S1 and S2 made from vinyl aromatic monomers and, between these, at least ore random soft block B/S made from vinyl aromatic monomers and from dienes, where the proportion of the hard blocks is above 40% by weight, based on the total block co-polymer.
The invention further relates to a process for preparing the block copolymers, and also to their use.
U.S. Pat. No. 4,939,208 describes linear, transparent styrene-butadiene block copolymers of the structure S1xe2x80x94B1xe2x80x94B/Sxe2x80x94S2. The polymerisation of styrene and butadiene in the presence of a Lewis base, in particular tetrahydrofuran (randomizer), gives the random copolymer block B/S. The length of the B/S segment depends on the amount of Lewis base.
EP-A 0 654 488 describes polymodal coupled styrene-butadiene block copolymers. The blocks B/S contain a styrene gradient (tapered block). By adding polar compounds, such as tetrahydrofuran, as randomizers, the random proportion in the blocks can be increased.
Polymerisation of styrene and butadiene in the presence of small amounts of tetrahydrofuran as randomizer gives a high proportion of homopolybutadiene blocks and a tapered transition to the polystyrene block. If the amount of tetrahydrofuran is increased, this gives butadiene-styrene copolymer blocks with some degree of random character it also sharply increases the relative proportion of 1,2 linkages in the polydiene (1,2-vinyl content). The high 1,2-vinyl content, however, impairs the thermal stability of the corresponding block copolymers and increases the glass transition temperature.
DE-A 19615533 describes an elastomeric styrene-butadiene block copolymer in which the relative proportion of 1,2 linkages in the polydiene is 15% and the proportion of the hard phase is from 1 to 40% by volume. The polymerization of the soft phase is undertaken in the presence of a soluble potassium salt.
Random copolymerisation of styrene and butadiene in cyclohexane in the presence of soluble potassium salts is described by S. D. Smith, A. Ashraf in Polymer Preprints 34 (2), 672 (1993) and 35 (2), 466 (1994). The soluble potassium salts mentioned comprise potassium 2,3-dimethyl-3-pentanolate and potassium 3-ethyl-3-pentanolate.
It is an object of the present invention to provide a glass-clear impact-modified polystyrene which has a balanced toughness/stiffness ratio and does not have the abovementioned disadvantages. In particular, the impact-modified polystyrene should have high intrinsic thermal stability and reduced thixotropy. It should also be compatible with styrene polymers.
We have found this object is achieved by means of block copolymers comprising at least two hard blocks S1 and S2 made from vinyl aromatic monomers and, between these, at least one random soft block B/S made from vinyl aromatic monomers and from dienes, where the proportion of the hard blocks is above 40% by weight, based on the total block copolymer, when the 1,2-vinyl content in the soft block B/S is less than 20%.
For the purposes of the present invention, vinyl Content is the relative proportion of 1,2 linkages of the dime units based on the total of 1,2, 1,4-cis and 1,4-traps linkages. The 1,2-vinyl content of the soft blocks is preferably from 10 to 20%, in particular from 12 to 16%.
Vinyl aromatic monomers which may be used for the hard blocks S1 and S2 or else for the soft blocks B/S are styrene, a-methylstyrene, p-methylstyrene, ethylstyrene, tert-butylstyrene, vinyl toluene or mixtures of these, preferably styrene.
Preferred dienes for the soft block B/S are butadiene, isoprene, 2,3-dimethylbutadiene, 1,3-pentadiene, 1,3-hexadienes or piperylene or mixtures of these, particularly preferably 1,3-butadiene.
The block copolymer is preferably composed exclusively of hard blocks S1 and S2 and also of at least one random soft block B/S, and does not contain any homopolydiene blocks B. Preferred block copolymers contain external hard blocks S1 and S2 having different block lengths. The molar pass of S1 is preferably from 5000 to 30000 g/mol, in particular from 10,000 to 20,000 g/mol. The molar mass of S2 is preferably above 35,000 g/mol. Preferred molar masses of S2 are from 5,000 to 15,000 g/mol.
Between the hard blocks S1 and S1 there may also be more than one random sot block B/S. Preference is given to at least 2 random soft blocks (B/S)1 and (B/S)2 having different proportions of vinyl aromatic monomers and therefore different glass transition temperatures. The block copolymers may have a linear or star-shaped structure.
The linear block copolymer used preferably has the structure S1xe2x80x94(B/S)1xe2x80x94(B/S)2xe2x80x94S2. The molar ratio of vinyl aromatic monomer to diene S/B in the block (B/S)1 is preferably below 0.25 and in the block (B/S)2 a preferably from 0.5 to 2.
The star-shaped block copolymers used preferably have a structure in which at least one arm of the star has a block sequence S1xe2x80x94(B/S) and one arm of the star has the block sequence S2xe2x80x94(B/S), or those in which at least ore arm of the star has the block sequence S1xe2x80x94(B/S)xe2x80x94S3 and at least one arm of the star has the block sequence S2xe2x80x94(B/S)xe2x80x94S3. S3 here is another hard block made from the vinyl aromatic monomers mentioned.
Most preferred for star-shaped block copolymers are structures in which at least one arm of the star has a block sequence S1xe2x80x94(B/S)1xe2x80x94(B/S)2 and at least one arm of the star has the block sequence S2xe2x80x94(B/S)1xe2x80x94(B/S)2 or those in which at least one arm of the star has the black sequence S1xe2x80x94(B/S)1xe2x80x94(B/S)2xe2x80x94S3 and at least one arm of the star has the block sequence S2xe2x80x94(B/S)1xe2x80x94(B/S)2xe2x80x94S3. The molar ratio of vinyl aromatic monomer to diene S/B in the block (B/S)1 is preferably from 0.5 to 2 and in the block (B/S)2 is preferably below 0.5.
The novel black copolymers may, for example, be formed by sequential anionic polymerisation, where as least the polymerisation of the blocks (B/S) takes place in the presence of a potassium salt. The potassium salt is generally used in a molar deficiency, based on the anionic polymerisation initiator. The molar ratio selected of anionic polymerisation initiator to potassium salt is preferably from 10:1 to 100:1, particular preferably from 30:1 to 70:1. The potassium salt used should generally be soluble in the reaction medium. Examples of suitable potassium salts are potassium alcoholates, in particular a potassium alcoholate of a tertiary alcohol having at least 7 carbon atoms. Particular preference is given to use of potassium 2-methylbutanolate, potassium 2,3-dimethyl-3-pentanolate, potassium 2-methylhexanolate, potassium 3,7-dimethyl-3-octanolate (potassium tetrahydrolinaloolate) or potassium 3-ethyl-3-pentanolate. The potassium alcoholates are obtainable, for example, by reacting elemental potassium, potassium/sodium alloy or potassium alkylates with the appropriate alcohols in an inert solvent.
It is useful for the potassium salt not to be added to the reaction mixture until the anionic polymerisation initiator has been added. In this way hydrolysis of the potassium salt by traces of aprotic contaminants can be avoided. The potassium salt is particularly preferably added just prior to polymerisation of the random soft block B/S.
Anionic polymerisation initiators which may be used are the usual mono- bi- or multifunctional alkali petal alkyl compounds, alkali metal aryl compounds or alkali metal aralkyl compounds. It is advantageous to use organolithium compounds, such as ethyl-, propyl-, isopropyl-, n-butyl-, sec-butyl-, tert-butyl-, phenyl-, diphenylhexyl-, hexamethyldi-, butadienyl-, isopropyl- or polystyryllithium, 1,4-dilithiobutane 1,4-dilithio-2-butene or 1,4-dilithiobenzene. The amount of polymerisation initiator needed depends on the desired molar weight. It is generally from 0.001 to 5 mol %, based on the total amount of monomers.
The polymerisation may be undertaken in the presence of a solvent. Suitable solvents are those aliphatic, cycloaliphatic or aromatic hydrocarbons which have from 4 to 12 carbon atoms and are usual for anionic polymerisation, such as pentane, hexane, heptane, cyclohexane, methylcyclohexane, iso-octane, benzene, alkylbenzenes, such as toluene, xylene or ethylbenzene, or decalin or appropriate mixtures. Preference is given to cyclohexane and methylcyclohexane.
The polymerisation may also be carried out without solvent in the presence of organyl metal compounds which slow down the rate of polymerisation, such as alkylmagnesium compounds, alkylaluminum compounds or alkyl zinc compounds.
Once the polymerisation has ended the living polymer chains may be capped using a chain terminator. Suitable chain terminators are protonating substances or Lewis acids, such as water, alcohols, aliphatic or aromatic carboxylic acids, or also inorganic acids, such as carbonic acid or boric acid.
Instead of adding a chain terminator once the polymerisation has ended, the living polymer chains may also be linked to give a star shave by polyfunctional capping agents, such as polyfunctional aldehydes, ketones, esters, anhydrides or epoxides. Symmetrical or asymmetrical star block copolymers whose arms may have the abovementioned block structures may be obtained here by coupling identical or different blocks. Asymmetrical star block copolymers are obtainable, for example, by preparing the individual arms of the star separately and/or by initiating more than once, for example initiating twice with the initiator divided in a ratio of from 2/1 to 10/1.
The novel block copolymers behave as glass-clear impact-modified polystyrene with a balanced toughness/stiffness ratio. The random soft block B/S with a low 1,2-vinyl content gives the novel black copolymers high intrinsic thermal stability.
The novel block copolymers also have good compatibility with other styrene polymers, and may therefore be processed to give polymer mixtures. The novel block copolymers or polymer mixtures may be used for producing fibers, forms of moldings.